This invention relates to a squirrel-cage rotor having a rotor core with slots and rotor bars embedded in the slots.
Generally, in the squirrel-cage rotor of the above-described type, a laminated rotor core is formed by laminating a number of steel sheets each having a plurality of slot forming punched portions previously formed in the outer circumference thereof by way of pressing or the like. The rotor bars with integral end rings are embedded in the slots by way of aluminum casting.
In accordance with the above-described construction, magnetic fluxes entering the rotor core from a stator via an air-gap during drive of the rotor contain harmonics. The harmonics in the magnetic fluxes cause a harmonic electromotive force in the rotor bars. Such a harmonic electromotive force acts as an abnormal torque against the rotor, and consequently induces a pulsating torque or causes vibration or noise.
Conventionally, the slots in which the rotor bars are embedded are skewed in order that undesirable influences of the harmonics may be restrained. In this method, when the steel sheets are laminated, the positions of the slot forming punched portions of the rotor are slightly deviated circumferentially of the rotor in turn. For example, the positions of the punched portions are deviated by one pitch of stator slots in sum. As a result, the phases of the electromotive force induced in the rotor bars by the harmonics are deviated slightly from portion to portion of one rotor bar. Consequently, the harmonics due to the whole electromotive force are canceled, and occurrence of the abnormal torque is restrained.
However, the following disadvantages are found in the above-described conventional construction. First, specific jigs are needed for adjusting the pitch of the rotor slots when the steel sheets are laminated with the slots skewed and the adjustment needs a lot of time. Particularly, when the slots are of the completely closed type, a lot of time is needed since the slot positions cannot be visually confirmed from the outer peripheral side in the state that the steel sheets have been laminated. Accordingly, the production cost of the rotor is increased. In skewing the slots, the skewing effect is reduced to a large extent unless an amount of skewing is properly set. A proper amount of skewing has not been established in theory and therefore, trial manufactures need to be made so that the proper amount of skewing is determined, resulting in increase in the production cost.
Second, when the rotor bars are usually embedded in the skewed slots of the laminated core, a metal such as aluminum needs to be cast into the slots. In this case, defect portions such as a gas pocket occur in stepped portions of the laminated steel sheets formed within the slots. Such defect portions unbalance the weight distribution of the rotor and therefore, the rotational stability is reduced, particularly, at the time of the high speed rotation.
Third, in the case of the conventional rotor with the skewed slots, arrangement and shape of the slots formed in both stator and rotor sometimes causes an abnormal torque called "position torque" depending upon positions of the stator and rotor relative to each other at starting. More specifically, when the amount of skewing is less than one pitch of the stator slots, the magnitude of torque which the rotor receives from the stator at starting changes in accordance with the position of the rotor. To restrain the position torque, the above-described amount of skewing is further increased to the value exceeding one pitch of the stator slots. Thus, the amount of skewing is increased in accordance with a stop position of the rotor, thereby restraining the position torque. However, when the amount of skewing is increased as described above, slot sections of the rotor are not effectively utilized. Consequently, the temperature of the rotor is increased or the characteristic thereof is lowered.
In order to obviate the above-described defects of the skewed rotor, it has been proposed to gain the skewing effect without actually skewing the slots at the time of lamination of the steel sheets. For example, Japanese Published Utility Model Reg. Application (Kokoku) No. 52-17045 discloses an opening or bridge portion of each slot deviated relative to the slot center line. In this case, however, the theoretical amount of deviation is not presented and the skewing effect cannot be found when the amount of deviation of each steel she et is improper. Thus, an optimum value of deviation needs to be learned from experiments when the above-described invention is actually applied, thereby increasing the design cost.